1. Field of the Invention
The present invention relates to a layer crystal structure oxide called the Aurivillius crystallographic group, a production method of the layer crystal structure, and a memory element using the same.
2. Description of the Related Art
Recently, development is actively conducted on nonvolatile memories comprising a ferroelectric thin film. Accordingly, bismuth strontium tantalate (Bi.sub.2 SrTa.sub.2 O.sub.9 ; hereinafter referred to as BiSTa) particularly attracts attention as a ferroelectric material comprising nonvolatile random access memories (ferroelectric random access memories: FeRAM) since fatigue caused by the polarization reversal is not present therein (C. A-Paz de Araujo, J. D. Cuchiaro, L. D. McMillan, M. C. Scott and J. F. Scott, Nature, 374 (1995) 627.; K. Amanuma, T. Hase and Y. Miyasaka, Appl. Phys. Lett., 66 (1995) 221.; S. B. Desu and D. P. Vijay, Master. Sci. and Eng., B32 (1995) 75., and the like).
The BiSTa is a so-called Aurivillius crystallographic group, which has been studied by various researchers so far (G. A. Smolenskii, V. A. Isupov and A. I. Agranovskaya, Soviet phys. Solid State, 3 (1961) 651.; E. C. Subbarao, Phys. Rev. 122 (1961) 804.; R. E. Newnham, R. W. Wolfe and J. F. Dorrian, Mater. Res. Bull., 6 (1971) 1029., and the like). The Aurivillius crystallographic group can be represented by a stoichiometric composition of [Bi.sub.2 O.sub.2 ].sup.2+ [Me.sub.m-1 R.sub.m O.sub.3m+1 ].sup.2-, wherein m represents an integer of 2 or more, Me represents at least one selected from the group consisting of sodium (Na), potassium (K), calcium (Ca), barium (Ba), strontium (Sr), lead (Pb), and bismuth (Bi), and R represents at least one selected from the group consisting of iron (Fe), niobium (Nb), tantalum (Ta), and tungsten (W).
Recently, a BiSTa thin film production with an MOCVD (metal organic chemical vapor deposition) method for the application in an FeRAM is reported, however, from the result of the X-ray analysis, a good quality film has not been obtained so far (T. Ami, K. Hironaka, C. Isobe, N. Nagel, M. Sugiyama, Y. Ikeda, K. Watanabe, A. Machida, K. Miura and M. Tanaka, Mater. Res. Soc. Symp. Proc., 415 (1996) 195.; T. Li, Y. Zhu, S. B. Desu, C-H. Peng, M. Nagata, Appl. Phys. Lett., 68 (1996) 616.).
For the application of the BiSTa in an FeRAM, the relationship between its composition and the electric characteristics is important. Various kinds of researches have been conducted for clarifying the relationship. For example, concerning polycrystalline BiSTa thin films produced by an MOD (metal organic decomposition) method, it is reported that a thin film with a good ferroelectivity can be obtained with a material composition containing bismuth more than the stoichiometric composition (H. Watanabe, T. Mihara, H. Yoshimori and C. A. Paz de Araujo, Jpn. J. Appl. Phys., 34 (1995) 5240). However, this indicates the relationship between the material composition and the electric characteristic, but not the relationship between the thin film composition and the electric composition. Furthermore, according to the report, the reason why a bismuth amount more than the stoichiometric composition in the material composition is preferable is concluded that finally the stoichiometric composition is obtained owing to the evaporation of the bismuth during the film formation process. Since bismuth is a highly volatile substance, this has been a common conventional view.
It is reported that the EPMA (electron probe microanalysis) of a polycrystalline BiSTa thin film produced by a sol-gel process with a material containing an excessive amount of bismuth and an insufficient amount of strontium with respect to the stoichiometric composition revealed that a composition region indicating the ferroelectivity contains an excessive amount of bismuth and an insufficient amount of strontium with respect to the stoichiometric composition (T. Atuki, N. Soyama, T. Yonezawa and k. Ogi, Jpn. J. Appl. Phys., 34 (1995) 5096; Y. Ito, M. Ushikubo, S. Yokoyama, H. Mtunaga, T. Atuki, T. Yonezawa and K. Ogi, Jpn. J. Appl. Phys., 35 (1996) 4295).
However, according to another research report, the polycrystalline BiSTa thin film accordingly produced contains residual impurities such as a metal bismuth, a bismuth alloy component, and a Bi oxide excluding BiSTa in the grain boundary (C. D. Gutleben, Y. Ikeda, C. Isobe, A. Machida, T. Ami, K. Hironaka and E. Morita, Mat. Res. Symp. Proc, 415 (1996) 201). That is, considering that the range of the measurement region of the EPMA (several .mu.m.phi. to 50 .mu.m.phi. beam diameter) remarkably larger than one grain particle size of the polycrystalline BiSTa thin film, it is impossible to analyze the composition in one grain particle of the polycrystalline thin film by the EPMA. Therefore, the above-mentioned analysis result by Atuki et al. contains the bismuth impurities so that it does not show the BiSTa composition ratio.
Another analysis of the composition of the polycrystalline BiSTa thin film by the ICP (inductively coupled plasma) is reported (T. Noguchi, T. Hase and Y. Miyasawa, Jpn. J. Appl. Phys., 35 (1996) 4900). This is also an analysis of the film as a whole containing the metal bismuth present in the grain boundary. That is, there is a certain limitation in grasping the relationship between the BiSTa composition and the electric characteristic by a polycrystalline BiSTa, and thus a research by a single crystal is needed.
However, concerning a single crystal of BiSTa, only very few crystallographic researches are reported including ones conducted by Newnham and Rae are reported (R. E. Newnham, R. W. Wolfe, R. S. Horsey, F. A. Diaz-Colon and M. I. Kay, Mater. Res. Bull., 8 (1973) 1183.; A. D. Rae, J. G. Thompson and R. L. Withers, Acta. Cryst., B48 (1992) 418). Among the two articles, one reported by Newnham deals with a substance prepared by substituting a part of strontium by barium, and thus it is not a pure BiSTa. Beside, the composition ratio of the starting material is not described precisely. In the Rae research, although the starting material has a constant ratio composition, a plate single crystal is obtained only in a two phase mixed state but a single phase synthesis is not achieved therein. Furthermore, the characteristics of the obtained single crystal are hardly analyzed in either research.
As to the other Aurivillius crystallographic group excluding the BiSTa, substantially no research has been conducted on a single crystal but only researches on Bi.sub.4 Ti.sub.3 O.sub.12 single crystal by Doriann and Burton are reported (J. F. Dorrian, R. E. Newnham, D. K. Smith and M. I. Kay, Ferroelectrics, 3 (1971) 17.; T. M. Bruton, Ferroelectrics, 7 (1974) 259).
Therefore, although the Aurivillius crystallographic group represented by the BiSTa attracts attention as a ferroelectric material, the relationship between the composition and the electric characteristic has not been known at all. Therefore, clarification of the relationship between the composition and the electric characteristic has a significant meaning in the application of these layer crystal structure oxide films in an FeRAM, and the like.
As methods for producing single crystals of the Aurivillius crystallographic group, a flux method and a TSSG method (top-seeded solution growth) can be presented.
In these method, a flux is added to the material for melting so as to grow a crystal from the liquid phase.
However, in the BiSTa production by the TSSG method by using bismuth oxide (Bi.sub.2 O.sub.3) as a self flux, since the melting point of the material added with the flux is as high as 1300.degree. C. or more, the bismuth oxide drastically evaporate by heating to the melting point such that the liquid surface cannot be seen, and thus it is difficult to grow a crystal. However, although there is a report on the production of Bi.sub.3 TiNbO.sub.9, which can be completely melted, among the Aurivillius crystallographic group by the pulling method. However, since a second substance is generated by the bismuth evaporation, and the second substance is taken into the crystal, a good quality crystal cannot be obtained (Daiku Okusakawa, Kenyu Honma, Masanobu Wada, Tohoku University Electric Communication Discussion Group Record, Vol. 3, No 2 (1974) 71). That is, the TSSG method involves a problem in that it is difficult to obtain a good quality crystal due to the bismuth oxide evaporation.
On the other hand, according to the flux method, although the BiSTa can be produced, since the BiSTa has the c face cleavage property, the crystal growth rate is low in the c axis direction, and thus only a thin plate crystal having a 1 to 2 mm square size can be obtained (Japanese Patent Application No. 8-283072). Further, there is a report on the production of Bi.sub.4 Ti.sub.3 O.sub.12, which is a incongruent melting compound of the Aurivillius crystallographic group by the flux method. However, again in this case, only a thin plate transparent crystal was obtained due to difficulty in growth in the c axis direction (Hatsuhiko Naito, Koichiro Sakata, Kenyu Honma, Gisaku Ohara, Titanium Barium Study Group Material No. XVI-93-649 (168) 174). That is, the flux method involves a problem in that only a thin plate crystal can be obtained to cause difficulty in handling and putting into actual practice due to the thinness.